In general, ABS resins, which are obtained by graft copolymerizing an aromatic vinyl monomer and a vinyl cyan monomer to a diene rubber polymer prepared by polymerizing conjugated diene monomers, have excellent impact resistance and processability, and are excellent in mechanical strength, heat deflection temperature, and colorability. Thus, ABS resins have been widely used in electric and electronic products, automobile parts, and office equipment. However, since rubber polymers used in preparation of ABS resins contain chemically unstable unsaturated bonds, there is a problem that rubber polymers are easily aged by ultraviolet light and thus the weather resistance of ABS resins is very weak.
To overcome such disadvantages, there has been proposed a method of adding a stabilizer capable of improving weather resistance when a resin composition is prepared by extruding an ABS resin, but the effect thereof was insufficient and the problem of being vulnerable to ultraviolet light was still not solved. Thus, a method, in which a polymer obtained by mixing and polymerizing a diene monomer and an acrylic monomer is used or a chemically more stable acrylic rubber polymer is used instead of a diene rubber polymer containing double bonds, has been proposed.
A representative example of a weather-resistant thermoplastic resin using an acrylic rubber polymer containing no unstable double bond is acrylate-styrene-acrylonitrile (ASA). Since ASA does not contain unstable double bonds in the polymer, it has excellent weather resistance, chemical resistance, and thermal stability, and thus ASA has been widely used in fields requiring such properties, for example, outdoor electric and electronic parts, building materials, farm equipment, ASA/ABS double layer sheets, profile extrusion, road signs, outdoor products, PVC for construction materials, leisure goods, sporting goods, and automobile parts.
In addition, as an emulsifier in preparation of the ASA copolymer and similar ASA resins, low molecular weight carboxylates, including potassium rosinate, sodium rosinate, sodium laurate, sodium oleate, potassium oleate, and potassium stearate, or sulfur-containing emulsifiers, including alkyl sulfosuccinic acid metal salt derivatives having 12 to 18 carbon atoms, alkyl sulfuric esters having 12 to 20 carbon atoms, and sulfonic acid metal salts, have been used.
However, in the case of such a low molecular weight emulsifier, the amount of the emulsifier to be added is relatively increased to sufficiently secure the stability of a latex. As a result, the emulsifier remains in the latex and a resin after polymerization, and thus gas is significantly generated on the surface of the resin during a high-temperature thermoforming process. This may reduce the thermal stability and surface properties of the resin. Recently, ASA resins have been applied to thin film products such as sidings, sheets, and co-extrusion films. Accordingly, there is growing demand for a technique of reducing the amount of gas generated in a molding process by reducing residues in a resin to improve thermal stability and appearance characteristics.